<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>
	<input type="text" placeholder="半径" class="a" />
	<input type="text" placeholder="周长" class="b" />
	<input type="text" placeholder="面积" class="c" />
	<script>
		var p = Math.PI;
		var btn = document.querySelector('button');
		var a = prompt('圆的半径是');
		var s = Math.pow(a,2) * p;
		var w = 2 * p * a;
		document.querySelector('.a').placeholder = a;
		document.querySelector('.b').placeholder = s;
		document.querySelector('.c').placeholder = w;
	</script>
</body>
</html>